1. Field of the Invention
The present invention relates to an apparatus used in an eye-refractometer device or an automatic lens meter and, more particularly, to an apparatus for measuring the refracting power of an optical system.
2. Related Background Art
As a method of objectively measuring the refracting power of an eye, a skiascopy is known as per, e.g., U.S. Pat. No. 4,353,625. A conventional apparatus adopting the skiascopy has a schematic structure shown in FIG. 11. The apparatus includes a projection system constituted by a light source 901, a lens 902 for collimating a light beam emitted from the light source 901, a chopper 903 for chopping the collimated light beam using a slit, and an image rotator 904 for rotating an image. An observation system is constituted by the image rotator 904 for rotating and returning light coming back from an eye to be examined (in other words, an image projected onto the fundus of the eye to be examined), and a lens 905 as a measurement optical system. A diaphragm 906 and a light-receiving unit 907 are arranged after the lens 905. The diaphragm 906 is arranged at a position conjugate with the fundus of the eye having reference refracting power, and plays a role to guide the shadow of a phase according to the refractive index of the eye to the light-receiving unit 907. The light-receiving unit 907 comprises two light-receiving elements 907u and 907d (FIGS. 12-14) which are vertically arranged to be separated by a predetermined interval.
The principle of measuring the refractive index by the conventional skiascopy will be described below with reference to FIGS. 12 to 14.
A fringe pattern is projected onto the fundus of an eye E to be examined by the above-mentioned projection system. The fringe pattern is moved in a predetermined direction at a predetermined speed.
When the refractive index of the eye E to be examined is normal, a fringe pattern portion formed at the same position Ea of the fringe pattern on the fundus of the eye is formed on the light-receiving elements 907u and 907d (see FIG. 12). Therefore, the light-receiving elements 907u and 907d are set in the same dark or bright state all the time. More specifically, when the light-receiving element 907u is set in a bright state, the light-receiving element 907d is also set in a bright state, and vice versa. In this state, an image formed on the light-receiving unit 907 simply repetitively flickers in correspondence with the dark and bright portions of fringes passing the position Ea (in this case, the fringe pattern is formed on the light-receiving unit 907, and never moves).
However, when the eye E to be examined suffers from myopia or hyperopia, an image corresponding to a position Eb is formed on the light-receiving element 907u, and an image corresponding to a position Ec is formed on the light-receiving element 907d. Therefore, the dark or bright states of the light-receiving elements 907u and 907d do not always coincide with each other (see FIGS. 13 and 14). Furthermore, when the fringe pattern formed on the fundus of the eye is moved (e.g., downward), the fringe pattern formed on the light-receiving unit 907 also moves in the same direction (in this case, downward; see FIG. 13) in the case of the myopia. Conversely, in the case of the hyperopia, the fringe pattern moves in the opposite direction (in this case, upward; see FIG. 14). The moving speed of the fringe pattern formed on the light-receiving unit 907 corresponds to the degree of myopia or hyperopia. Therefore, the refractive index of the eye can be measured by measuring the time after a certain fringe passes through the light-receiving element 907u until it reaches the light-receiving element 907d (see FIG. 15). The basic principle of the skiascopy has been described.
Upon measurement of the refracting power of an eye, however, not only the hyperopia or myopia but also astigmatism must be taken into consideration. More specifically, since the refracting power of an eye has directivity, the directivity must be taken into consideration to achieve precise measurement. For example, when an eye to be examined suffers from astigmatism, a fringe pattern formed on the fundus of the eye is rotated before it reaches the light-receiving portions, and crosses a portion between the light-receiving elements 907u and 907d in an oblique state, as shown in FIG. 16. Therefore, in order to obtain information about astigmatism, angles corresponding to maximum and minimum times between the light-receiving elements must be detected. For this purpose, the conventional apparatus detects the angles corresponding to the maximum and minimum times by rotating the image using the image rotator 904.
However, in the conventional apparatus, in order to obtain data associated with the refractive index (i.e., a diopter (a reciprocal number of a focal length) defined as a function of the spherical power, the cylindrical power, and the cylindrical axis degree as two-dimensional quantities), the image rotator as a means for converting scalar data into vector data must be rotated through at least half a revolution during measurement.
However, the image rotator 904 normally comprises a large prism asymmetrical about the rotational axis, and a mirror, and has a very poor balance. Therefore, to rotate the image rotator in each measurement imposes a considerable load on the structure of the apparatus. Also, it is difficult to increase the rotational speed, and this makes it difficult to reduce measurement time. In the manufacture, it is very difficult to adjust, e.g., the rotational axis of the image rotator 904. Such limitations on the structure and the manufacture increase the cost of the apparatus. In this manner, the image rotator impairs measurement precision, makes the manufacture difficult, and increases cost..
For this reason, various conventional systems without using any image rotator have been proposed. For example, as disclosed in U.S. Pat. No. 4,526,451, light beams are incident on a chopper in two orthogonal directions, and are time-divisionally extracted using these beams as horizontal and vertical scanning beams. However, in this system, the chopper must be manufactured with very high precision. In order to achieve such high precision, the chopper must be manufactured by grinding a metal, thus posing a problem of cost. In this system, two light sources must be used. However, since the light sources individually have different light amount distributions and characteristics, the variations of the light sources adversely influence the measurement values, thus disturbing high-precision measurement. For these reasons, this system is not adopted in a practical use in place of the image rotator system.
Recently, a requirement for instantaneously measuring the refracting power of an eye in, e.g., a refracting power correction operation, an intraocular lens operation, and the-like has become more common. In order to meet such a requirement, the apparatus main body must be rendered highly compact to improve mobility. However, as described above, it is difficult for the conventional apparatus to achieve a compact structure.